How To Evaluate Laser Beam Quality? (Part 1)

Laser beam quality is an important technical index of laser, and the characteristics of laser are evaluated from the qualitative aspect. However, for a long time, there has been no exact definition of beam quality, and no standard measurement method has been established, which has brought inconvenience to scientific research and application.

In 1988, A.E. Seigman introduced a dimensionless quantities-beam quality factor parameter, which described the laser beam quality more scientifically and reasonably and was adopted by the ISO/TC172/SC9/WG1 standard draft in 1991. The research and measurement of laser beam quality factor has also become a research hotspot in recent years.

The methods commonly used to evaluate the beam quality are:

The original diffraction limit multiple factors, Streier ratio, circumference-energy ratio, M2 factor, or its reciprocal K factor (beam transmission factor), the definition of various beam quality corresponds to different application purposes, reflecting the beam quality emphasis is also different. The quality of the beam should be evaluated according to the specific application purpose.

Beam quality is an important way to evaluate the characteristics of a laser in terms of quality, which plays an important role in the design, manufacture, detection, and application of the laser. It is of great significance to discuss how to perfect the universality of beam quality factors, analyze the factors affecting beam quality and control them.

For analyzing the accuracy of long-range laser ranging and laser altimetry, measuring the beam quality is an important link, especially the energy distribution of the beam and the size of the divergence Angle and beam width, the influence on the ranging echo is more obvious, and the laser beam quality factor can reflect the quality of the ranging laser and improve the measurement accuracy.

1. Original diffraction limits multiple factors

The original diffraction limit multiple factors is defined as the ratio of the far-field divergence Angle of the measured actual beam to the far-field divergence Angle of the ideal beam, and its expression is β=θ/θ0 (1)

The far-field emission Angle θ is determined by the asymptotic formula θ=limz→∞/w(z)z, and w(z) is the width of the light spot and z is the corresponding position of the light spot. In the case that the paraxial approximation and the aperture diffraction are negligible, the beam width w(z) in free space satisfies the following transmission equation:

w2(z)=w20+(M2)2 (λ/(πw0))2 (Z-Z0)2 (2)

Where z0 is the location of waist w0. For an ideal Gaussian beam, the spot width w(z) is defined by the width at the light intensity max value 1/e2, and the defined spot size contains 86.5% of the total power of the Gaussian beam.

The original diffraction limit factor β is mainly suitable for evaluating the laser beam just emitted from the laser resonator, and can reasonably evaluate the near-field beam quality. It is a static performance index describing the beam quality of the laser system, and does not consider the effects of atmospheric scattering, turbulence, and thermal halo on the laser.

In the laser countermeasure weapon system, the β is mainly determined by the βL of the laser weapon system exiting the beam and the βd of the beam through the beam direction

β2=β2L+β2D (3)

It provides a basis for evaluating the beam quality of high-energy laser weapon systems.

The measurement of the β value depends on the accurate measurement of the far-field divergence Angle of the beam, due to the factors of the laser itself and the influence of many factors in the laser beam transmission process, so that the intensity distribution of the far-field beam contains more high-order spatial frequency components, the intensity of the laser is attenuated by using CCD to measure the width of the spot, it is difficult to detect the high-order component of the spot, the relative spatial intensity distribution is very good It is difficult to reflect the higher-order component of the spot, and the obtained β value cannot truly reflect the energy loss caused by the higher order dispersion. Its accurate measurement requirements for the detection system are high, and it is not suitable for evaluating the beam transmitted over a long distance.

2. Annular energy ratio

The annular energy ratio, also known as the target surface (or barrel) power ratio, is defined as the root of the ratio of the ideal annular energy (or power) of the light spot within the specified size to the actual annular energy (or power) of the light spot within the same size. Its expression is

BQ=P Ideal P measured

Or BQ=EE real ideal test (4)

The BQ value is used to evaluate the far-field beam quality for energy delivery and coupling applications, combined with the energy concentration of the beam on the target.

The difference between the BQ value and β value is that it contains atmospheric factors, and it is a comprehensive index to describe beam quality from the perspective of engineering application and damage effect and a dynamic index of laser weapon systems affected by the atmosphere. The BQ value is directly related to the beam quality and power density, which is the reflection of the energy concentration, and has a very practical significance for the study of the energy coupling and damage effect between the high-intensity laser and the target.

The description of the spatial distribution of light intensity in a single barrel is insufficient, and the selection of the ideal beam must be clearly defined. From the perspective of engineering application, the solid plane wave with the size of the main mirror of the laser weapon launching system is selected as the ideal beam with a relatively simple and practical method.

For "hard destruction" laser weapons, the standard size should be as small as possible, requiring a higher peak power density, the standard size to the main diameter of the main mirror of the launching system D corresponding to the diffraction limit size is more appropriate, it contains 84% of the total energy emitted by the system, for "soft kill" laser weapons, requires a higher energy share in the target surface range, a higher average power density The specification size can be selected as the size of the destruction target.

The BQ value is often measured by different limited hole energy measurement methods and detection systems that can measure the absolute energy distribution in space, requiring a strong light array detector or target disk instrument that can directly receive high-energy laser.

It is often used to evaluate the quality of intense laser beams, but because high-power lasers, such as hydrogen fluoride (HF), deuterium fluoride (DF), and oxygen-iodine chemical lasers (COIL), generally adopt a non-stable cavity structure, the output beam is not a Gaussian beam, there are some uncertainties in measuring the quality of the laser beam generated by non-stable cavity lasers.

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